SURFACE REFLECTANCE AND UNDERWATER
DOWNWELLING IRRADIANCE IN ALQUEVA
RESERVOIR, SOUTHEAST PORTUGAL
M. Potes, R. Salgado, M. J. Costa, M.
Morais, D. Bortoli and I. Kostadinov
Institute of Earth Sciences (ICT)
Évora, Portugal
Proambiente S.c.r.l., Bologna, Italy
Outline
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Introduction
Data
Validation of satellite data
Water surface reflectance
Underwater downwelling irradiance
Summary
Next step
Acknowledgments
Why Lakes/Reservoirs ?
Lakes are an important component of
the land surface, influencing the
weather at different scales. Some
regions can be highly influenced by the
presence of lakes:
• • The boreal zone (10% of the area of
Finland)
• • Eastern Africa and American Great
Lakes
• • In many regions (example of the
Mediterranean basin) dams and
reservoirs have been constructed
Lake Cover fraction
in ECMWF
Lakes in
Finland
and
Karelia
region
Importance of Lakes/Reservoirs
(Example)
• ECMWF IFS with FLAKE
Sensitivity of 48-hour T2m
forecasts (valid at 00 UTC)
for LAKE compared with
NOLAKE for summer.
• In summer the cooling
effect is pronounced, due
to incoming radiation
that is stored in the lakes.
•
• Lakes can release more
latent heat than dry land
(Balsamo et al., 2012)
Alqueva reservoir
Alentejo Region:
Köppen classification: Csa
Annual precipitation: 571,8 mm
Number of days above 30ºC: 77.1
Surface area of 250 km2
Gates were closed in 2002
Data
SATELLITE
ATMOSPHERIC
FIELD
LABORATORY
FieldSpec UV/VNIR
A Portable spectroradiometer from ASD - FieldSpec UV/VNIR was
acquire in 2008 by Evora Geophysic Centre from University of Évora
(http://www.cge.uevora.pt/en).
 325 – 1075 nm range




Absolute or relative measurements of light energy
3 nm spectral resolution at around 700 nm
1, 10, 25 and 180 degrees of view angle
17 ms to several minutes of integration time
FieldSpec UV/VNIR measurements
 Spectral water surface reflectance over Alqueva reservoir
• Validation of atmospheric correction of satellite remote sensing
reflectance
• Review of spectral absorption bands of biologic quantities
 Underwater spectral downwelling zenith (ir)radiance at Evora
Municipal Swimming complex and Alqueva reservoir
• Attainment of underwater ir(radiance) profiles until 3 m depth
• Estimation of spectral attenuation coefficient of the water column
GOAL
• Implementation of remote sensing algorithms to daily monitor (or
close) all surface area of Alqueva reservoir in terms of physical
and biological quantities.
Atmospheric correction of satellite data
L1 : Radiation scattered by the atmosphere
L2 : Reflected radiation from the viewed pixel
L3 : Radiation reflected by the neighborhood and
scattered into the view direction (adjacency effect)
60
Radiance [ w / (m2sr m) ]
TOA (L1+L2+L3)
50
Surface (L2)
40
Atmosphere and
adjacency (L1+L3)
30
20
10
0
400
500
600
Wavelength(nm)
700
800
Satellite and in situ water reflectance
- FieldSpec
- MERIS L1 + 6S
(atmospheric correction)
- MERIS L2
R = 0.84
RMSE = 0.0073
Water reflectance and Chlorophyll a
Chlorophyll a has
special absorption
bands that change
the spectrum through
its concentration
allowing spectral
remote sensing
detection
Water vs Petalite reflectance
It’s a mineral composed
by silica
The target here isn´t the boat!
It is the dark green colored water!
Biological algorithms
The results of this work
indicate that the methodology
proposed allows the regular
and inexpensive water quality
monitoring,
in
terms
of
chlorophyll and cyanobacteria
concentration.
Potes et al., 2011
Underwater Equipment
A portable FieldSpec UV/VNIR (ASD, Inc) is used to measure
spectral irradiance coupled to a protector and directional frame
by an optical cable which guide the light from a the tip to the
fieldspec. A hemispherical tip (180º FOV) was built to have
measurements independent of solar zenith angle.
FieldSpec
UV/VNIR
Optical
cable
180º FOV
Underwater apparatus developed by
the team
• Portability
• Limited to clear sky days. A profile (3 m
depth) takes in average 4 minutes and during
this period the atmospheric condition should be
the same.
Downwelling irradiance profile ->
Attenuation coefficient
Preisendorfer, 1958
Attenuation coefficient outcome
K = 1.0 m-1
This is spectral attenuation coefficient measured in
different water types, in several field campaigns,
with a portable fieldspec coupled to an optical
fiber and a radiance/irradiance underwater
receptor.
Potes et al., TellusA,
2013
K = 6.1 m-1
FLake model (Mironov et al., 2010) represents
better the daily cycle of water temperature with
attenuation coefficient of 6.1 m-1.
Potes et al., HESS, 2012
Tests in Evora Swimming Complex
This is the reservoir !
Tests in Evora Swimming Complex
Solar zenith angle = 18,24º
All spectrum decreasing
irradiance with depth.
Measurements in Alqueva Reservoir
Irradiance and Attenuation Coefficient
Preisendorfer, 1958
Attenuation coefficient and water type
Alqueva Reservoir K PAR
(400-700 nm) between
0.85 and 1.46 m-1
Pool K PAR = 0.2 m-1
Pure water K PAR ≈ 0.05 m-1
Attenuation coefficient and turbidity
K PAR
(m-1)
Turbidity
(FNU)
0.79
10.56
0.58
8.89
0.48
8.42
0.51
8.41
Next Step
• Use the attenuation coefficients obtained in
Alqueva reservoir together with satellite data
and develop an algorithm capable to estimate
this coefficient.
• Test the algorithm in other lakes and reservoir
for global estimation of this coefficient with a
possible database in mind.
Acknowledgments
• The work is co-funded by the European Union through the
European Regional Development Fund, included in the COMPETE
2020 (Operational Program Competitiveness and
Internationalization) through the ICT project (UID / GEO / 04683 /
2013) with the reference POCI-01-0145-FEDER- 007690 and in
ALENTEJO 2020 (Programa Operacional Regional do Alentejo)
through the ALOP project with the reference ALT20-03-0145FEDER-000004.
• The work was also supported by FCT postdoc grant
SFRH/BPD/97408/2013.
OBRIGADO!
GRACIAS !
References
Preisendorfer, R. W. 1959. Theoretical proof of the existence of characteristic
diffuse light in natural waters. Jour. Mar. Research, 18, 1-9.
Potes, M., Costa, M. J., Silva, J. C. B., Silva, A. M., e Morais, M., 2011. Remote
sensing of water quality parameters over Alqueva reservoir in the south of
Portugal. Int. J. Remote Sens., 32:12, 3373-3388.
Balsamo, G., Salgado, R., Dutra, E., Boussetta, S., Stockdale, T. and Potes, M.
2012. On the contribution of lakes in predicting near-surface temperature
in a global weather forecasting model. Tellus, 64A, 15829.
Potes, M., Costa, M. J., Salgado, R. 2012. Satellite remote sensing of water
turbidity in Alqueva reservoir and implications on lake modeling. Hydrol.
Earth Syst. Sci., 16, 1623-1633.
Potes, M., Costa, M. J., Salgado, R., Bortoli, D., Serafim, A. and Le Moigne, P.,
2013. Spectral measurements of underwater downwelling radiance of
inland water bodies. Tellus A, 65, 20774.